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nin Research; Schlossberger, H. G., Kochen, W., Linzen, B.,
Steinhart, H., Eds.; de Gruyter: Berlin, New York, 1984; p 115.
Werner, G. Eine HPLC-Methode zur Bestimmung von Tryptophan in Futtermitteln und Getreide. Landwirtsch. Forschung. 1986, 39, 1. Williams,A. Kp.; Hewitt, D.; Buttery, P. J. A CollaborativeStudy on the Determination of Tryptophan in Feedingstuffs. J. Sei. Food Agric. 1982, 33, 860. Wong, W. S. D.; Osuga, D. T.; Burcham, T. S.; Feeney, R. E. Determination of Tryptophan as the Reduced Derivative by
Acid Hydrolysis and Chromatography. Anal. Biochem. 1984, 143, 62.
Zahnley, J. C.; Davis, J. G. Effect of High Tyrosine Content on the Determination of Tryptophan in Protein by the Acidic Ninhydrin Method. Biochem. J. 1973, 135, 59. Received for review May 30,1989. Revised manuscript received October 10, 1989. Accepted October 23, 1989. Registry No. Tryptophan, 73-22-3.
Investigation of Contents of Some Elements in Soil and Apricots by Atomic Absorption Spectrometry Mustafa Demir,' Seref Gucer, and Turkan Esen Faculty of Arts and Science, Department of Chemistry, Inonu University, Malatya, Turkey
In this work, the relationship between the concentrations of six metals (Cd, Zn, Mn, Pd, Cu, Fe) in acetic acid extracts (2.5%) of soil and their respective concentrations in apricots grown in the same soil has been investigated. Soil extracts and apricots have been analyzed by atomic absorption spectrometry with injection or loop sample introduction. Metals were preconcentrated with activated carbon by using ammonium pyrrolidinedithiocarbamate (APDC) as a complexing agent a t pH 6.
It is well-known that sodium, potassium, calcium, magnesium, and iron are essential for human life and are supplied by feeding. Improvements in analytical techniques show that copper, chromium, cobalt, manganese, selenium, molybdenum, and zinc are also essential for human life and take part in enzyme structure. It is also well-known that mercury, cadmium, lead, and talium, which are toxic to human life, are also taken through feeding. Many countries accept threshold levels for such elements. For the analysis of food samples, the main problem is preparation of samples for measurement. Food samples can be decomposed either by wet digestion or by dryashing methods, depending on the elements being analyzed. For volatile elements like Cd, Zn, T1, etc., the wet digestion method is preferred (Stoeppler and Brandt, 1979; Verloo, 1982;Hoenig and Borger, 1983; Demir e t al., 1985). A wide range of extractants have been used in soil analysis to suit many purposes. For agriculture, the total trace-element content of a soil has become less important than the extractable contents. The selective chemical extraction of soil components with various extractants has been outlined (Pickering, 1981). In soil testing, it is important to select a solvent that can extract many elements simultaneously. Some trace elements in the extractants may be in lower concentrations; thus, many elements cannot be directly determined. In these cases, preconcentration steps and/or different sampling techniques have been applied (Pederson e t al., 1980; Roberts e t al., 1976; Berndt and Messerschmidt, 1982). When soil is polluted with toxic elements, there is no
possibility of elimination but such a situation may be improved by controlling the soil p H by liming, thus leading to irreversible fixation. EXPERIMENTAL SECTION Reagents. All chemicals were analytical reagent grade unless otherwise stated. Deionized water was used throughout. Stock solutions (1000 mg/L) were used to prepare standard solutions of metals by diluting with 2 M HNO,. Ammonium pyrrolidinedithiocarbamate solution and activated carbon suspension were prepared as stated elsewhere (Gucer and Demir, 1987). Buffer (pH 6) was prepared by mixing 5 mL of glacial acetic acid with 117 g of ammonium acetate and diluting the resultant mixture to exactly 1 L. Apparatus. A Perkin-Elmer Model 400 atomic absorption spectrometer (without background corrector)assembled with a 5-cm air/acetylene burner head was used for atomic absorption measurements. Single-element hollow cathode lamps (S/ C Juniper) were used as a radiation source. Wavelengths were selected from the instrumentation manual, and a 2.0-nm slit width was used. Acetylene and air flow rates of 2.6 and 9.6 L/ min, respectively,were used throughout the measurements. Analyses of Fe, Cu, and Mn have been done by a conventional flame atomic absorption spectrophotometric method in which 100p L sample solutions are aspirated directly into the flame. A loop device (Berndt and Messerschmidt, 1979, 1982) was used as a sample introducer (10 wL) for measurements of Zn, Cd, and Pb. When the loop device was used, a silica tube (6-mm i.d. and 47-mm length) was used as a collector and positioned ca. 5 cm above the burner head. A IO-WLsample solution was injected on the platinum loop and dried by passing 3-A ac current through the loop (ca. 20 s). Then, the loop was transported under the window of collection tube through which the light was passing, and shock heating was applied for 0.2 s elec-
0 1990 American Chemical Society
J. Agric. Food Chem., Vol. 38, No. 3, 1990 727
Elements in Soil and Apricots
7
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Figure 1. Soil-apricot relations of Cd, Zn, Mn, and Pb (numbers indicate regions of soil and apricot samples taken).
trically. Peak height measurements were used to calculate absorbance values. Procedure. Apricots were collected from four different regions of Malatya, Turkey, and seven samples were taken from each region. In order to prevent microbiological activity, samples were sulfured by holding them in SO, vapor and they were dried in air. After being dehydrated at 110 O C in an oven for 48 h (15-20% with respect to air-drying), 5 g of pulverized sample was decomposed with mixtures of concentrated nitric acid, concentrated sulfuric acid, and 30% hydrogen peroxide (10/5/1 ratio) (Demir et al., 1985), evaporated to dryness, heated 30 min with 150 mL of deionized water, and filtered through blueband filter paper. Ammonia was added to the filtrate to pH 6, then 20 mL of buffer solution (pH 6), 20 mL of APDC solution, and 4 mL of activated carbon suspension were added, and the resultant mixture was shaken for 1 h with a shaker (Hitofrig, 160 rpm). The mixture was filtered through blue-band filter paper, and the residue was dried in an oven for 1 h at 110 O C . After addition of 2 mL of concentrated nitric acid to the residue and evaporation to dryness, the residue was shaken with 5 mL of 2 M nitric acid. The solution was separated by centrifugation (5000 rpm) and used for AAS measurements with either direct injection into the flame (100 pL) or loop introduction into the heated silica tube (10 pL). Soil acetic acid extracts were prepared and analyzed as stated elsewhere (Gucer and Demir, 1987). Calibration graphs used in soil analysis and apricot analysis were drawn from the data in which the same analytical procedures had been applied as with the samples analyzed.
RESULTS AND DISCUSSION For the analysis of soil extracts by AAS, different sample introduction techniques have been applied depending on the amounts in extracts. For example, for copper, iron, and manganese, the extraction solution is directly aspirated to the flame whereas loop sampling with a collection tube has been applied for zinc. On the other hand, loop with collection tube, following a preconcentration step with activated carbon, has been applied for cadmium and lead systems. In this way, the detection limit has been increased about 20 times by applying a preconcentration step with activated carbon. An additional, about 10 times, increase in detection limit has been reached by using a collection tube followed by preconcentration and loop sampling. Relative standard deviations of absorption measurements are given in Table I. It is well-known that plants take up not only essential elements necessary for their normal development but also other ones if these are present in growth medium in a sufficiently mobile form. Accumulation of nonessential elements influences plant production quantitatively as well as qualitatively. The relationship between the content of elements in soil, their concentrations in plant tissues, and growth is a complex phenomena. In most cases, mineral contaminants accumulate in the upper layer of the soil where
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Demir et al.
5. 0
Table I. Relative Standard Deviations of Absorption Measurements element CU" Fen Mn" Znb Cdb Pbb
3
3
20
3;
-1 0
1.e in Soil
45
50
55
(nig/:
